For example, substrates or steel sheets being continuously being transferred (at high speed) may be coated with a coating material such as a metal vapor under a vacuum using a known vacuum deposition method.
In a vacuum deposition process, a solid (phase) or liquid (phase) coating material is evaporated to form vapor (gas) by various heating-evaporating methods and deposited on a steel sheet to coat the steel sheet.
Examples of such vacuum deposition methods for continuously coating substrates (steel sheets) include a thermal evaporation method and an electron beam evaporation method.
In addition, electromagnetic levitation evaporation methods have recently been researched and developed for high-speed deposition.
In an electromagnetic levitation evaporation method, a coating material is surrounded by an electromagnetic coil, and a high-frequency alternating current (AC) current is applied to the electromagnetic coil from a high-frequency power source to levitate and heat the coating material by an AC electromagnetic field. When compared to an existing method of generating a metal vapor in a crucible, a large amount of metal vapor may be generated with less thermal loss, and a substrate or a steel sheet being continuously transferred (even at high speed) may be coated with the metal vapor.
As described above, to coat a steel sheet being continuously transferred under a vacuum, a heating apparatus (evaporating apparatus) is used for generating a coating vapor, and a coating material (to be evaporated for coating) is supplied for continuous coating.
A coating material may be supplied using a solid state (solid) supply method or a liquid state (liquid) supply method according to the state of the coating material, and examples of the liquid state supply method include a mechanical method, a height difference method, or a pressure difference method.
Examples of the mechanical method (liquid state supply method) include a piston method (US Patent Publication No. 2005-0229856), a magnetohydrodynamics (MHD) pump method using electromagnetism (Korean Patent Laid-open Publication No. 2007-0015923), and a screw supply method (Japanese Patent Laid-open Publication No. 2010-189739).
In addition, an example of the height difference method is disclosed in Korean Patent Laid-open No. 2009-0074064, and an example of the pressure difference method is disclosed in Japanese Patent Application Laid-open Publication No. S55-154537.
The liquid state supply methods disclosed in the above-mentioned patents have a common problem: equipment erosion (abrasion) caused by temperature or chemical properties of a supplied liquid material.
In an exemplary solid state (solid) supply method, a solid wire is supplied. In this case, however, the temperature of a vapor in an electromagnetic coil may be relatively low.